Field of the Invention
The present disclosure relates to a viewing angle switchable back light unit in which the general mode and the privacy mode in a liquid crystal display can be selected, and more particularly, to a back light unit in which the display information can be suggested within a specific viewing angle in a liquid crystal display by using an ultra thin light guide film.
Discussion of the Related Art
Nowadays, due to the characteristics of light weight, slim thickness, low consumption electric power, the liquid crystal display device (or, LCD) is widely applied more and more. The LCD is applied to the portable computer such as note book PC, the official automation devices, the audio/video devices, the advertizing display device for indoor or outdoor, and so on. The transmittive type LCD, the most used type, represents the video images by modulating the luminescence of the light incident from the backlight unit by controlling the electric field applied to the liquid crystal layer.
Typically, there are two types of back light unit, the one type is the direct type and the other is the edge type. The direct type back light unit has a structure in which a plurality of optical sheets and a diffusion plate are stacked under the liquid crystal panel and a plurality of light sources are disposed under the diffusion plate. FIG. 1 is a cross sectional view illustrating a structure of the liquid crystal display having the direct type back light unit including the light emitting diode (or, LED) array as the light source according to the related art.
The direct type back light unit DBLU includes a light source disposed under a liquid crystal panel LCP and radiating the back light directly to the liquid crystal panel LCP. The light source may be a thin fluorescent lamp. Otherwise, as shown in FIG. 1, the light source may be the LED array LEDAR having a lower power consumption and enhanced luminescence. The LED array LEDAR is disposed in a matrix manner on the bottom surface of the case CASE. The case CASE may be installed at the cover bottom CB. In some cases, the case CASE may be omitted, and the LED array LEDAR may be disposed at the cover bottom CB directly. On the LED array LEDAR, a diffusion plate DIFF is disposed. The diffusion plate DIFF diffuses the back light from the LED array LEDAR to provide the evenly scattered back light over the light incident surface of the liquid crystal panel LCP. Between the diffusion plate DIFF and the liquid crystal panel LCP, a plurality of optical sheets OPT may be disposed. The optical sheets OPT include one or more prism sheet, one or more diffusion sheet, and/or a dual brightness enhancement film (or DBEF). The prism sheet converses the scattered and/or diffused back light by the diffusion plate DIFF to the liquid crystal panel LCP for enhancing the brightness of the back light. The diffusion sheet diffuses again the conversed back light by the prism sheet over the liquid crystal panel LCP to having evenly distributed luminescence.
A guide panel GP wraps and/or surrounds the side surfaces of the liquid crystal panel LCP and the direct type back light unit DBLU and supports the liquid crystal panel LCP by inserting between the liquid crystal panel LCP and the optical sheets OPT. The cover bottom CB wraps and/or surrounds the case CASE and the bottom surface of the direct type back light unit. On the bottom surface of the case CASE having the LED array LEDAR, a reflective sheet REF is disposed to reflect the back light leaked from the diffusion plate DIFF and/or the optical sheets OPT to the liquid crystal panel LCP. The top case TP surrounds the upper edge of the liquid crystal panel LCP and the side surface of the guide panel GP.
In the interim, the edge type back light unit may have thinner thickness than the direct type back light unit. Currently, the liquid crystal display devices have the LED light source rather than the fluorescent lamp. Especially, due to the easy installation of the light source, the edge type back light unit, in which LED light sources are disposed at the side surface of the liquid crystal panel, is more widely applied.
Hereinafter, referring to FIG. 2, we will explain about the edge type back light unit. FIG. 2 is a cross sectional view illustrating a structure of the liquid crystal display having the edge type back light unit including the light emitting diode array as the light source according to the related art.
Referring to FIG. 2, the edge type back light unit comprises a cover bottom CB, a light guide LG disposed at a bottom surface of the cover bottom CB, and light source disposed between the side surface of the light guide LG and the cover bottom CB and providing the back light to the side surface of the light guide LG. The light source may be a thin fluorescent lamp. Otherwise, as shown in FIG. 1, the light source may be the LED array LEDAR having a lower power consumption and enhanced luminescence. The light source may be disposed at the side surface of the light guide LG using the installing means like housing. The light guide LG receives the back light from the LED array LEDAR and refracts the direction of the back light as being perpendicular to the light incident surface of the liquid crystal panel LCP. Between the light guide LG and the liquid crystal panel LCP, a plurality of optical sheets OPT is disposed. The optical sheets OPT include one or more prism sheet, and one or more diffusion sheet for scattering and/or diffusing the back light from the light guide LG. To enhance the brightness and/or luminescence, the optical sheets OPT may further include a dual brightness enhancement film (or DBEF).
The guide panel GP wraps and/or surrounds the side surfaces of the liquid crystal panel LCP and the edge type back light unit, and supports the liquid crystal panel LCP by inserting between the liquid crystal panel LCP and the optical sheets OPT. Between the cover bottom CB and the light guide LG, a reflective sheet REF is disposed to reflect the back light leaked from the diffusion plate DIFF and/or the optical sheets OPT to the liquid crystal panel LCP. The top case TP surrounds the upper edge of the liquid crystal panel LCP and the side surface of the guide panel GP.
As mentioned above, the liquid crystal display, one example of non-self luminance display, should have the back light unit. The back light unit would be configured to suggest the back light evenly distributed over the whole surface of the liquid crystal panel. Therefore, various optical devices and/or means for transforming a light from the point light source or the line light source to a light from the plane light source. Further, considering the optical characteristics and structures of these optical devices and/or means, the back light unit would have some thickness. Even though the flat type displays including the liquid crystal display are remarkably thinned than the conventional display like CRT (i.e., cathode ray tube), more advanced technology for developing new back light unit suitable for much thinner and lower power consumption display device is required.
The light system according to the related art mentioned above may be applied in the back light unit in the non-self luminance display such as the liquid crystal display. The back light unit is disposed at the rear side of the liquid crystal display for providing the back light to the front side where the observer is located. The back light unit is designed as to radiate the back light to the front direction with evenly distributed luminances.
In most cases, the liquid crystal display provides the video information to all people who can look the front surface of the display. However, in some cases, the liquid crystal display may be designed to provide its video information to specific person positioned in the just front side of the display. For examples of the television sets or the outdoor information displays, it is preferable that the displays are designed to provide good video quality to the public people as many as possible. In that case, the back light unit would have the wide viewing angle of 180 degree and even luminance distribution characteristics over the whole surface of the display. On the contrary, for the personal portable display, it is preferable that the display may be selected as to operate in the privacy mode in which the display can provide the video information just to the person who own the device. The most back light unit cannot be selectively controlled as to be the privacy mode (operating in the narrow viewing angle) or the general mode (operating in the wide viewing angle).
The only way to select between the privacy mode and the general mode is to attach the privacy optical film on the display surface or disattach the privacy optical film from the display surface. More detail, when the privacy optical film is disposed on the front surface of the liquid crystal display, the display can be used in the privacy mode in which the display information can be observed only by the person located just in front of the display. In that case, if the display has certain additional device such as the touch panel, the privacy optical film is added on the additional device so that the additional device may not operate normally. Further, it is somewhat annoying or not easy for user to manage the privacy optical film.